In the manufacture of sheet steel, sheet aluminum and other sheet metal products, paper, plastics and other sheet materials, work rolls or cylinders (referred to hereinafter as “rolls”) are employed in the finishing process, for example, in a steel mill, a paper mill or the like. The roll shape and surface are textured to a high degree of accuracy. Traditionally, the roll surface is textured by processes such as shot blasting, electrical discharge texturing (EDT), electron beam texturing, laser texturing and other methods known to those skilled in the art.
Electrical discharge texturing, also referred to as spark elctroerosion or electroblast texturing, is a known process for texturing the working surface of a roll. The process involves the computer controlled spark erosion of the roll surface. The sparks are caused to pass between electrodes and the roll surface through a dielectric fluid. The roll can be partially or completely immersed in the dielectric fluid or the electrode ends can be dammed off and flooded with dielectric fluid and/or the dielectric fluid can be flushed through the electrodes, the fluid exiting at the electrode ends near the roll surface. The electrodes can have a positive polarity or a negative polarity. When the polarity is negative, the spark passes from the electrodes to the roll surface and, when the polarity is positive, the spark passes from the roll surface to the electrodes. Controlled electrical current is generated and discharged through the dielectric fluid producing the ionic charges or sparks that thermally explode into or out of the roll surface. This charge melts the extreme outer layer of the roll, forming microscopic craters proportional in size and spacing to the energy supplied and the frequency of generation. The result is an extremely precise, predictable and controllable texture on the roll surface.
The rolls referred to herein are electrically conductive metal rolls that can be made from various ferrous or non-ferrous materials or combinations of both. In practice, the rolls are predominantly made with steel but they may optionally have an outer shell of copper or nickel. They also may be made from cast iron, tool steels, various alloys, titanium, tungsten carbide, combinations thereof and the like.
Rolls are textured for various reasons for use in steel mills, paper mills and the like. For example, rolls used in steel mills to roll steel sheet that is to be painted cannot be too smooth or the paint adhesion and appearance on the rolled steel sheet will have high variances of quality. Texturing of the rolls provides the rolled steel sheet with a textured surface to which paint adheres and flows better and reduces quality variance. This texturing is conventionally performed by an EDT apparatus wherein a bank of electrodes is moved adjacent the roll surface and/or the roll is moved parallel to the electrode faces as the roll is rotated. EDT machines have electrode positioners each of which moves its associated electrode generally along its central axis toward and away from the roll to control the voltage drop and, accordingly, the electrical ionization process. In some EDT machines the electrodes are positioned about 10° below the center line of the roll on one side and about 10° above the center line on the other side so that they can be thrown out of the way if the roll surface hits them during rotation.
Electrode positioners ensure that each electrode is properly spaced with respect to the roll surface so as to be close enough to generate momentary sparks through dielectric fluid but not too close so as to generate a continuous current flow without any sparks to perform the texturing operation.
EDT machines for commercial applications are large and expensive. In most cases they are also inflexible as to diameters and lengths of rolls that can be textured on a particular machine. It is an object of the present invention to make a smaller, less expensive EDT machine that can be used to texture a broad range of roll diameters and lengths. A further object of the invention is to make an EDT machine that is suitable for use in low volume production or research and development applications while maintaining the ability to texture the roll surface to the same or improved degree of accuracy as the large commercial scale machines.